1. Field of the Invention
The present invention relates to a diffractive optical element used for a lens in an optical system, the optical system, and an optical apparatus having the optical system.
2. Description of the Related Art
For a diffractive optical element used for a lens in an optical system, it is known to adhere two diffraction gratings closely to each other and to properly set a material and a grating height of each diffraction grating so as to provide high diffraction efficiency over a wide wavelength range. When a light flux enters this diffracting optical element that includes grating surfaces and grating wall surfaces, the incident light flux is reflected on or diffracted by the grating wall surface, causing unnecessary light (flare). Japanese Patent Laid-Open Nos. (“JPs”) 2003-240931 and 2004-126394 propose a diffractive optical element that includes an absorption film on the grating wall surface so as to restrain the unnecessary light (flare) on the grating wall surface. JPs 2004-13081 and 2005-62717 adhere two diffraction gratings closely to each other and provide a thin film so as to improve the adhesion property on the interface. JP 2009-217139 discloses a calculation of diffraction efficiency utilizing the rigorous coupled wave analysis (“RCWA”).
For the diffractive optical element used for the lens in the optical system, especially problematic and unnecessary light is unnecessary light caused by a total reflection on an interference between a high refractive index medium and a low refractive index medium, of a light flux incident at an obliquely incident angle (off-screen light incident angle) different from a designed incident light flux. However, JPs 2003-240931, 2004-126394, 2004-13081, and 2005-62717 do not care about this problem, or provides an insufficient effect of restraining the unnecessary light.